Inhibitory forms of killer cell Ig-like receptors (KIR) are important negative regulators of human natural killer (NK) cell activation. They maintain NK cell tolerance through recognition of MHC class I molecules (HLA-A, -B, and -C) on the surface of normal cells in the body. KIR transduces inhibitory signals through the recruitment of SHP-1 and SHP-2 tyrosine phosphatases. Since NK cell activation is controlled by a balance between activating and inhibitory receptors, the surface expression level of KIR is a key determinant of the NK cell activation potential. While a great deal of effort has been directed toward understanding the mechanism by which KIR transduces inhibitory signals, very little is known about the post-translational mechanisms regulating surface expression, endocytosis, and plasma membrane trafficking of these receptors. Improved understanding of the mechanisms regulating KIR surface trafficking and expression could provide novel therapeutic targets to promote NK cell activation toward tumors and virus infected cells by reducing the surface expression levels of KIR. We will define the molecular mechanisms regulating inhibitory KIR trafficking by addressing the following specific aims. Aim 1 will assess the role of ligand in regulating surface expression of inhibitory KIR. Here we will test the hypothesis that ligand engagement impacts upon the membrane trafficking of inhibitory KIR. Aim 2 will elucidate the molecular mechanisms mediating endocytosis and degradation of inhibitory KIR. We hypothesize that certain molecular interactions control these pathways, and our experiments will test their impacts on surface receptor expression. Aim 3 will examine the role of the b arrestin 2 adaptor protein on surface trafficking of inhibitory KIR. We hypothesize that this adaptor promotes surface expression of KIR and stabilizes interaction of the receptor with SHP-1 and SHP-2 phosphatases. PUBLIC HEALTH RELEVANCE: Natural killer (NK) cells can attack tumors and virus-infected cells, and their activation state is controlled by a balance of signals from activating and inhibitory receptors. Inhibitory killer cell Ig-like receptors (KIR) are key regulators on human NK cells that establish tolerance toward normal cells of the body, but surprisingly little is known about the mechanisms that maintain their surface expression. The studies proposed in this application will characterize the molecular regulation of KIR membrane trafficking to develop strategies for reducing their surface expression and thereby potentiating NK cell responses toward tumors and viruses.